1. Field of the Invention
The present invention relates to a platen assembly for supporting thereon printing paper printed in a printer.
2. Statement of the Prior Art
A conventional platen includes a solid shaft of a metal material and an elastic cylinder body of a rubbery elastomer, which is mounted around said shaft. Printing paper is held around the elastic cylinder body and is printed with a printing head.
However, such a conventional platen has an increased weight and inertia due to the elastic cylinder body having a large thickness relative to the solid shaft. A problem with such a platen has thus been that its responsiveness during rotational driving is so poor that not only is the printing speed slow but difficulty is encountered in controlling its rotational position. Another problem has been that when the temperature of the platen rises during continuous printing over an extended period of time and the rubber or other member expands so as to increase the diameter of the elastic cylinder body, there is a decrease in the distance between the platen and the printing head, which in turn causes deterioration of the quality of the printing and the feed pitch of the platen to be increased too much to be in alignment with that of a tractor, which in turn causes troubles such as jamming.
One way to solve such problems of the prior art platen is to change the solid shaft to a pipe shaft. In other words, the pipe shaft is used to reduce the increase in the weight of the platen, and an elastic cylinder body having a reduced thickness is provided therearound, whereby it is possible to obtain a platen of reduced weight and inertia, and it is possible to reduce the variation in the outer diameter of the rubber due to temperature changes. Such a platen is disclosed in, e.g., Japanese Utility Model Laid-Open No. 62-25556.
However, such a platen has a structure in which plug members are mounted at both ends of the pipe shaft by force fitting, welding or other means so as to rotatably support the platen on a frame by a thin supporting shaft inserted axially through the plug members. Thus, not only is there an increase in the number of parts involved, but high fit accuracy is required for force fitting, welding or other means. Besides, it is required to effect axial alignment of the supporting shaft after force fitting or welding, resulting in an uneconomical increase in the number of steps during assembling.